<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        div{
            width: 100px;
            height: 50px;
            background:blue;
            line-height: 50px;
            text-indent: 20px;
            color: white;
        }
       span{
            width: 0px;
            height: 0px;
            display: inline-block;
            border: 5px solid transparent;
            border-top: 5px solid black;
            position: relative;
            top: 2.5px;
       }
       .box:hover span{
           border-top: 5px solid transparent;
           border-bottom: 5px solid black;
           top: -2.5px;
       }
    </style>
</head>

<body>
   <div class="box">
       导航
       <span></span>
   </div>
</body>

</html>